Sealed lead-acid cells and batteries, for many applications, have significant advantages in comparison to the use of conventional, flooded lead-acid cells and batteries. Such cells and batteries, sometimes termed "VRLA" cells and batteries (i.e., valve-regulated, lead-acid), utilize a safety valve (e.g., a Bunsen valve) to maintain the desired internal pressure for an efficient oxygen recombination cycle.
Some applications where such sealed cells and batteries are used are termed stationary battery applications. In such applications, such cells and batteries are maintained at a full state-of-charge and in a ready-to-use condition, typically by floating at a constant preset voltage. Stationary cells and battery applications include use for stand-by or operational power, including telecommunications, utilities, emergency lighting for commercial buildings, stand-by power for cable television systems, and uninterruptible power supplies.
Such uninterruptible power supplies concern, for example, systems which back-up computers and communication networks. Having a reliable uninterruptible power source accommodates the orderly shutdown of computers when there is a sudden interruption in the primary power source, typically during an electrical power outage. Such an uninterruptible power source will also accommodate short, or intermittent, losses in power. In the event of a power interruption, the uninterruptible power source is subject to a rapid, and sometimes deep, discharge.
Another potential application for sealed lead-acid cells and batteries is a variety of motive power applications in which an array of cells or batteries provides the motive power for vehicles ranging from Class 1 to Class 3 trucks, various automated guided vehicles, mining vehicles and also railroad locomotives. The performance requirements for motive-powered vehicles are quite different from the performance requirements for stationary battery power sources. In stationary applications, the depth of discharge in service is relatively shallow, and the number of discharges is small, as most batteries are in float service. In direct contrast, motive power applications require relatively deep depths of discharge to be achieved on a continuous cycling basis over a period of time. Indeed, a common requirement for Class 1-3 trucks is that, in an 8-hour shift, the cell or battery assembly must be capable of delivering an 80% depth of discharge and that performance is required for about 300 cycles per year with a useful service life under those conditions of 4 or 5 years.
A common problem encountered by such VRLA cells and batteries is the integrity of the seals over time as a result of grid growth which occurs in service. As has been especially common at the positive terminal, grid growth occurs as a battery grid corrodes over time, hence causing the battery terminal to move outwardly relative to the battery cover. This relative rise causes stress on both the required terminal-cover seal, as well as the requisite container-cover seal. As the container and cover are commonly made of plastic, each was known to fail in various ways, such as by fracturing, cracking at welds, and so forth. Failures such as these have resulted in the leakage of electrolyte from the cells and batteries and has also resulted in failure of the cell to operate properly. This well-known problem is particularly acute in sealed lead-acid cells and batteries because relatively high internal temperatures in service can be reached. Such relatively high temperatures lead to an increase in the rate of grid corrosion which can result in significant grid growth. Further, what can occur in service is deflection of the positive terminal post which can result in a possible loss of connection to the adjacent stationary terminal post. Indeed, such grid growth can result in the buckling of a positive plate resulting in loss of capacity as well as possible shorting and the like.
This problem is not only well-known, but also has commanded substantial attention. A wide variety of attempted solutions have been proposed. Various solutions are thus described in U.K. Patent 2,026,761A, U.S. Pat. Nos. 4,455,356 to Barrette, Jr.; 4,467,021 and 4,898,795 to Stocchiero.
There are several other issues and concerns which complicate the design of sealed lead-acid cells. More particularly, to provide a satisfactory terminal-cover seal for sealed lead-acid cells, it has generally been found necessary to utilize a lead or lead alloy bushing which is embedded in the cover that is, then, in turn, welded or fused to the terminal. This design approach can be relatively expensive, and amenability to automatic production, while achieving satisfactory quality control, can be difficult.
Still further, design considerations need to address problems such as avoiding, in service, plate edge shorting, uneven separator saturation levels and electrolyte stratification. Insuring that the cell possesses satisfactory shock and vibration resistance must also be taken into account in designing sealed lead-acid cells.
These design issues and considerations often can be exacerbated as the size of the individual cells required for the particular application is increased. Indeed, relatively large capacity cells raise additional design issues, such as insuring that satisfactorily reliable electrical connections between the plates and the respective terminals are retained. Yet another design issue concerns satisfactory alignment of the cell element in the jar or container during assembly.
These design issues are of substantial concern because a variety of applications for sealed lead-acid cells, such as, for example, stand-by power in telecommunications, make the application of cells having extremely high capacities desirable. Indeed, in this connection, it becomes desirable to provide individual sealed lead-acid cells having capacity of 2,000 Ampere Hours and indeed up to 3,000 Ampere Hours capacity, or even greater. Considerable difficulty in achieving commercial production with acceptable quality control is encountered when designing cells of such relatively large capacities.
Indeed, despite the substantial amount of prior work in this field, there still exists a need for a design for sealed lead-acid cells which effectively satisfies the many issues and concerns as discussed herein.
Accordingly, it is a primary object of the present invention to provide a sealed, VRLA lead-acid cell that at least minimizes, if not eliminates, the problems associated with positive plate growth during service. A related and more specific object provides such a cell which may be used to essentially prevent the growth of the positive plate in a direction toward the cover and terminal during service.
Another object is to provide such a sealed cell having a design particularly well suited for cells having relatively high capacities.
A still further object is to provide a cell which eliminates the need for a bushing in the cover and is more amenable to large scale and/or automated production.
Yet another object is to provide a cell in which precise positioning of the cover and terminal during assembly is unnecessary.
A further object is to provide a sealed cell amenable for large sized cells which provides improved performance, minimizing the problems due to shock and vibration resistance and plate shorting.
A still further object is to provide such a cell that minimizes the problems due to uneven separator saturation levels and eliminates electrolyte stratification issues.
Other objects and advantages of the present invention will be apparent as the following description proceeds, taken with the accompanying drawings. The present invention will be described in conjunction with certain preferred embodiments; however, it should be appreciated that it is not intended to limit the invention to the embodiments disclosed. Rather, it is intended to cover all alternative and equivalents to these embodiments and to the claimed invention. For example, while the present invention will be described in conjunction with sealed lead-acid cells and batteries, it should be appreciated that the present invention is equally applicable to use with any cell or battery wherein plate growth during service results and must be accommodated.